Threaded joints are widely used for connecting steel pipes (may alternatively simply referred to as pipes hereafter) used in oil production equipment such as OCTG. In related art, in order to connect steel pipes used for prospecting and producing oil or gas, standard threaded joints specified in the API (American Petroleum Institute) standard are typically used.
Nowadays, since the depth of crude oil wells and natural gas wells is increasing, and the number of vertical wells, horizontal wells, directional wells, and the like is increasing, the drilling/production environment becomes more demanding. Development of wells in severe environments such as ocean and polar region is also increasing. Thus, there are diversified requirements for performance of threaded joints such as compression resistance, bending resistance, and seal performance of external pressure. For this reason, so-called premium joints, which are high-performance special threaded joints, are more frequently used, and requirements for performance of the premium joints are also increasing.
The premium joint generally has a tapered thread, a metal-to-metal seal, and a torque shoulder. The tapered thread is an important part for tightly securing the joint for pipes. The metal-to-metal seal is used to reliably maintain sealability by metal-to-metal contact of a box component and a pin component with each other. The box component forms a female side of the threaded joint and the pin component forms a male side of the threaded joint. The torque shoulder serves as a shoulder surface that functions as an abutment during tightening of the joint.
FIGS. 3 to 5 are schematic diagrams of a related-art coupling type premium joint for OCTG, illustrating longitudinal sectional views of the cylindrical threaded joint. The threaded joint includes a pin 3 as a male member and a box 1 as a female member facing the pin 3.
The pin 3 has a male thread 7 on the outer surface thereof and a nose 8. The nose 8 is a non-threaded portion provided at a tip end of the pin 3 adjacent to the male thread 7. The nose 8 has a metal-to-metal seal (may alternatively be referred to as a pin seal) 11 on the outer circumferential surface thereof and a torque shoulder (may alternatively be referred to as a pin shoulder) 12 on the end surface thereof.
The box 1 facing the pin 3 has on the inner surface thereof a female thread 5, a metal-to-metal seal (may alternatively referred to as a box seal) 13, and a torque shoulder (may also be referred to as a box shoulder) 14, which are respectively screwed together with or in contact with the male thread 7, the metal-to-metal seal 11, and the torque shoulder 12 of the pin 3.
When pipes are installed in wells, the pipes are usually rotated during the work. When drilling in a horizontal or inclined direction, the pipes are rotated with the middle thereof bent and repeatedly subjected to tensile and compression forces while being rotated. In order to address this, structures having resistance for a compression load are proposed. For example, Patent Literature 1 proposes a structure in which deformation of the pin seal is limited within an elastic region in a compressed state by setting the length of a seal lip of the pin to equal to or greater than 140 times the length of the gap in a direction of the axis of pipe between male and female threads, which are screwed together.